Page 251 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

esophagus for only 5 to 10 s (only a small
Deglutition
amount remains). 2. pH clearance. The pH of
The upper third of the esophageal wall consists the residual gastric juice left after volume
of striated muscle, the rest contains smooth clearance is still low, but is gradually increased
muscle. During the process of swallowing, or during each act of swallowing. In other words,
deglutition, the tongue pushes a bolus of food the saliva that is swallowed buffers the re-
into the throat (! A1). The nasopharynx is re- sidual gastric juice.
flexively blocked, (! A2), respiration is in-
hibited, the vocal chords close and the epiglot- Vomiting
tis seals off the trachea (! A3) while the upper
esophageal sphincter opens (! A4). A peristal- Vomiting mainly serves as a protective reflex
tic wave forces the bolus into the stomach but is also an important clinical symptom of
Nutrition and Digestion (receptive relaxation) mediated by VIP- and nausea, increased salivation and retching
conditions such as intracranial bleeding and
(! A5, B1,2). If the bolus gets stuck, stretching
tumors. The act of vomiting is heralded by
of the affected area triggers a secondary per-
istaltic wave.
(! C). The vomiting center is located in the
The lower esophageal sphincter opens at
the start of deglutition due to a vagovagal reflex
medulla oblongata within the reticular forma-
tion. It is mainly controlled by chemosensors
NO-releasing neurons (! B3). Otherwise, the
of the area postrema, which is located on the
reflux of aggressive gastric juices containing
chemosensory trigger zone (CTZ). The blood-
pepsin and HCl.
brain barrier is less tight in the area postrema.
10 lower sphincter remains closed to prevent the floor of the fourth ventricle; this is called the
Esophageal motility is usually checked by measur- The CTZ is activated by nicotine, other toxins, and
ing pressure in the lumen, e.g., during a peristaltic dopamine agonists like apomorphine (used as an
wave (! B1–2). The resting pressure within the emetic). Cells of the CTZ have receptors for neu-
lower sphincter is normally 20–25 mmHg. During re- rotransmitters responsible for their neuronal control.
ceptive relaxation, esophageal pressure drops to The vomiting center can also be activated indepen-
match the low pressure in the proximal stomach dent of the CTZ, for example, due to abnormal
(! B3), indicating opening of the sphincter. In stimulation of the organ of balance (kinesia, motion
achalasia, receptive relaxation fails to occur and food sickness), overextension of the stomach or intestines,
collects in the esophagus. delayed gastric emptying and inflammation of the
Pressure in the lower esophageal sphincter is abdominal organs. Nausea and vomiting often occur
decreased by VIP, CCK, NO, GIP, secretin and pro- during the first trimester of pregnancy (morning sick-
gesterone (! p. 234) and increased by acetylcholine, ness) and can exacerbate to hyperemesis gravidarum
gastrin and motilin. Increased abdominal pressure leading to vomiting–related disorders (see below).
(external pressure) also increases sphincter pressure During the act of vomiting, the diaphragm re-
because part of the lower esophageal sphincter is lo-
cated in the abdominal cavity. mains in the inspiratory position and the
abdominal muscles quickly contract exerting a
Gastroesophageal reflux. The sporadic reflux high pressure on the stomach. Simultaneous
of gastric juices into the esophagus occurs contraction of the duodenum blocks the way
fairly often. Reflux can occur while swallowing to the gut; the lower esophageal sphincter
(lower esophageal sphincter opens for a then relaxes, resulting in ejection of the stom-
couple of seconds), due to unanticipated pres- ach contents via the esophagus.
sure on a full stomach or to transient opening of
the sphincter (lasts up to 30 seconds and is part The sequelae of chronic vomiting are attributable to
reduced food intake (malnutrition) and the related
of the eructation reflex). Gastric reflux greatly loss of gastric juices, swallowed saliva, fluids and in-
reduces the pH in the distal esophagus. testinal secretions. In addition to hypovolemia, non-
Protective mechanisms to prevent damage to respiratory alkalosis due to the loss of gastric acid
+
the esophageal mucosa after gastroesophageal (10–100 mmol H /L gastric juice) also develops. This +
is accompanied by hypokalemia due to the loss of K
reflux include 1. Volume clearance, i.e., the in the vomitus (nutrients, saliva, gastric juices) and
238 rapid return of refluxed fluid to the stomach urine (hypovolemia-related hyperaldosteronism;
via the esophageal peristaltic reflex. A refluxed ! p. 180ff.).
volume of 15 mL, for example, remains in the
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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